This invention relates to the manufacture of composition board by extrusion and, more particularly, to an improved agitation mechanism for insuring the uniform dispersion of the composition material throughout the extrusion mold.
The manufacture of composition board by an extrusion process is well known in the art. Typical composition board is formed from a mixture of fine ligno-cellulose particles combined with a suitable resin binder. The mixture is metered from a large hopper into a feed chamber where a reciprocating piston forces the mixture into a heated mold. The mold is open at an outlet end opposite to the inlet end where the piston is situated so that successive compressive forces applied to the mixture by the reciprocating piston results in the continuous molding of composition board. Typical apparatus for manufacturing composition board in accordance with afore described process is disclosed, for example, in U.S. Pat. No. 3,229,009, the disclosure of which patent is hereby incorporated by reference herein.
A typical composition board which may be manufactured by a machine of the type described in the aforementioned patent is a continuous 40 inches wide sheet with 13/16 inch ribs every 2 inches across the width of the board on one side thereof. To maintain uniform density of the composition board in both the flat panel portions and the ribs, it is necessary to distribute evenly the constituent materials prior to it being forced into the mold by the piston. To accomplish this, a plurality of oscillating fingers are provided in the feed chamber above the reciprocating piston. These fingers are spaced along the width of the mold with a pair of fingers above the position of each of the ribs. These fingers are adapted to oscillate in opposite directions. It has been found that the oscillation of the fingers must be timed relative to the stroke of the reciporcating piston in order to maintain uniform board density. In particular, the fingers oscillate through a selected angle, suitably approximately 60.degree. of rotation in opposite directions, "dead center" of the oscillation being in a vertical orientation with extremes of 30.degree. angular displacement to either side. It has been found that at approximately the time the reciprocating piston is fully retracted and ready to move forward to force the mixture into the mold, the fingers should be at their extremities of oscillation at opposite ends from the vertical orientation. The fingers should move to the opposite point of maximum angular displacement before the piston enters the mold and should stay in that position for the remainder of the forward piston movement. The reverse direction of oscillation of the fingers occurs during the reverse travel of the piston. Thus, the agitation mechanism must be precisely coordinated with the movement of the reciprocating piston. Prior attempts to do this reliably have not been entirely successful. For example, in the aforedescribed patent the movement of the agitating fingers is controlled by a complex eccentric cam arrangement. However, such cams wear out relatively quickly. It has also been found that a mechanical linkage between the piston and the agitating fingers is unreliable in that it is extremely difficult to properly adjust the timing between the movement of the piston and the agitating fingers.
Another important aspect of the invention is the discovery that by varying the time at which oscillation of the fingers occurs relative to the movement of the piston the density of the legs can be controlled.